[Gluster-users] GlusterFS performance, concurrency and I/O blocking

[Mohit Anchlia]

directory listing over 50 million files will definitely slow things
down if environment is not setup to handle that kind of IO. I am
thinking blocking could be due to network throttling, disk spins while
random IO is happening. Were you able to look at the network
utilization during this time period? You can install iftop to monitor
network utilizaiton. Also, there is a bug related to small files that
causes performance issues.

Bug - http://bugs.gluster.com/show_bug.cgi?id=2869

Definitely do some benchmark and see your IO requirements. This will
help you tune your system. But in general directory listings are slow
and consumes network too. In addition watch iostats and look for await
and %util.


On Tue, Aug 23, 2011 at 11:17 AM, Ken Randall <rushian85 at gmail.com> wrote:
> Hi everybody!  Love this community, and I love GlusterFS.
>
> All that, despite being burned by it, likely due to my own failures.  Here's
> the scenario where I got burned, and my guesstimates on why they happened.
>
> We run a popular .NET-based web app that gets a lot of traffic, where people
> build websites using our system.  The long short of it is, we tested,
> tweaked, tested some more, over a full month.  After we deployed it to
> production, we saw performance take a dive into the dumpster.  We had to
> revert back fairly quickly.
>
> The obvious blame is in our testing.  We load tested the system many, many
> times over the course of an entire month, but with a narrow range of test
> scenarios.  The wide range of live production traffic proved to render our
> testing moot.  We tucked our tail between our legs and are researching tools
> that will let us play back life traffic to serve as a better simulation.  In
> our earlier load-testing we were able to achieve many multiples of our peak
> traffic, but again it wasn't realistic traffic.
>
> Before I get to my suspicion of what's happening, keep in mind that we have
> 50+ million files (over hundreds of thousands of directories), most of them
> are small, and each page request will pull in upwards of 10-40 supporting
> assets (images, Flash files, CSS, JS, etc.).  We also have people executing
> directory listings whenever they're editing their site, as they choose
> images, etc. to insert onto the page.  We're also exporting the volume to
> CIFS so our Windows servers can access the GlusterFS client on the Linux
> machines in the cluster.  The Samba settings on there were tweaked to the
> hilt as well, turning off case-insensitivity, bumping up caches and async
> IO, etc.
>
> It appears as if GlusterFS has some kind of I/O blocking going on.  Whenever
> a directory listing is being pieced together, it noticeably slows down (or
> stops?) other operations through the same client.  For a high-concurrency
> app like ours where the storage backend needs to be able to pull off 10 to
> 100 directory listings a second, and 5,000 to 10,000 IOPS overall, it's easy
> to see how perf would degrade if my blocking suspicion is correct.  The
> biggest culprit, in my guess, is the directory listing.  Executing one makes
> things drag.  I've been able to demonstrate that through a simple script.
> And we're running some pretty monster machines with 24 cores, 24 GB RAM,
> etc.
>
> I tried as many tuning permutations as possible, only to run into the same
> result.  Jacking the cache-size, the io-thread-count to 64, etc. certainly
> helped performance, but continued to exhibit this blocking behavior.  I also
> made sure that each web server accessing the GlusterFS backend was talking
> to its own GlusterFS client, in the hopes of increasing parallelization.
> I'm sure it helped, but not enough.  It's nowhere close to the concurrency
> and performance of a straight-out Windows share.  (I realize the overhead of
> a clustered file system will have less perf than a straight share, but we
> saw a drop of performance as load increased, in the order of magnitude
> range.)
>
> Am I way off?  Does GlusterFS block on directory listings (getdents) or any
> other operations?  If so, is there a way to enable the database equivalent
> of "dirty reads" so it doesn't block?
>
> Ken
>
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